Up to a hundred thousand children in Britain whose asthma is not controlled by blue Ventolin inhalers are also given Salmeterol, a longer-acting inhaler drug.

But for one in every seven child sufferers the extra inhaler, commonly marketed as Seretide, may not work because they carry a gene which makes it ineffective, a study has found.

Failure to treat asthma with effective drugs causes the condition to grow worse, with an increase in wheezing and coughing and a higher risk of attacks.

A simple saliva test could identify which children carry the offending gene and allow doctors to prescribe an alternative such as Montelukast (also known as Singulair), a pill which is less effective for most children but much better for those who do not respond to Salmeterol.

Prof Somnath Mukhopadhyay of Brighton and Sussex Medical School, who led the study, said: “We should try to get some advice from the Department of Health or Asthma UK on the kind of advice we should be giving mothers and GPs, this is something we really need.

“If you have a child with Asthma and you give them a medicine that doesn’t work, it is making it worse. That is unacceptable in terms of treatment strategy because you are increasing the child’s risk of having school absence or hospital admissions.”

When children are diagnosed with asthma they are typically given a blue Ventolin inhaler to provide instant relief from attacks, but some need to use them so frequently that doctors also prescribe a steroid inhaler or, where this does not work, Salmeterol for longer-acting treatment.

Salmeterol works by binding to a molecule in the body called the beta-2 receptor, but one in seven asthma sufferers has a genetic mutation in the molecule which makes the treatment less effective.

The researchers studied 62 children carrying the mutation who regularly used steroid inhalers but had still missed school or needed hospital treatment for their asthma.

Half of the children was given Salmeterol inhalers and half took Montelukast pills over the course of a year. Those given Montelukast had a better quality of life, wheezed and coughed less, and relied less on their blue “reliever” inhalers.

At the start of the project a third of the children needed to use their relievers every day, but a year later this had halved among those using Montelukast, according to the study published in the Clinical Science journal.

Prof Mukhopadhyay said: “For almost every symptom we were looking at there was a significant difference and it was always in favour of Montelukast.

“We have for the first time shown that personalised medicine can work in the field of children’s asthma.”

There are also fears that the same patients may be responding worse to their blue inhalers because they use a similar type of drug, but further studies are needed to examine any possible link, he added.

The findings are unlikely to apply to adults because child asthma is very different to that experienced later in life.

Malayka Rahman of Asthma Research UK said: “This exciting area of research has the potential to lead to the tailoring of better treatments for an individual based on their own genetic make-up, ultimately keeping more people out of hospital and preventing unnecessary asthma deaths in the long term.”

Prof Stephen Holgate, MRC Clinical Professor of Immunopharmacology at Southampton University, added: “This is a wonderful example of stratified or personalised medicine working its way into practice.

“The gene-based test is not yet available to doctors, but should become so if larger trials are equally positive.”

A spokesman for GlaxoSmithKline, which manufactures Seretide, said: "The results of this small study raise interesting questions around how different patients can gain the most benefit from their medicines and it warrants further research.

"It is important to ensure that children continue to control and manage their asthma and we’d encourage anyone who has questions about the medicines their children take to talk to their doctor."